Microprocessor controlled tuning systems are being widely used in television receivers. One approach to such tuning systems is to use a microprocessor to control a PLL that corrects for drift in the local oscillator phase thereby insuring that the local oscillator is always precisely in phase with a fixed frequency crystal oscillator. Previously, such electronic tuning systems have had the shortcoming that they were programmed for proper tuning only under the condition that the received signal was at one of the nominal carrier frequencies specified by the FCC for television channels. There are occasions, such as arise in connection with CATV and MATV systems and with video games, when the signal is not at one of the nominal picture carrier frequencies, and as a result such a system does not produce proper tuning.
It is possible to design the microprocessor controlled PLL tuning system to permit the programming of offsets in the local oscillator frequency from the nominal value. In one such system, an entry from a keyboard causes the microporcessor to produce an output to a programmable frequency divider in the PLL which determines the nominal oscillator frequency. The tuner intermediate frequency is sampled to provide information to the microprocessor as to whether the tuner local oscillator needs to be adjusted to accommodate a frequency offset in the received signal. The microprocessor uses this information to generate an output to the programmable frequency divider which determines the amount of frequency offset of the local oscillator frequency away from the nominal value. There are occasions which have been encountered in practice with certain CATV systems, and some MATV systems, in which television signals on adjacent channels are converted to frequencies differing from the nominal value by the same amount. One example is the case of the so-called "constant interval" CATV system in which the carrier frequencies of both channels 5 and 6 are increased by 2 MHz as compared with the nominal values. This places the resultant sound carrier of channel 5 at 0.5 MHz above the nominal frequency of the channel 6 picture carrier. When channel 6 is selected, the tuning system as described up to this point never acquires the displaced channel 6 picture carrier 2 MHz above the nominal value. Instead, it locks to the displaced channel 5 sound carrier. In order to avoid this, if is necessary to provide the tuning system with a mechanism for beginning the search procedure again, but with a greater initial frequency offset.
A specific example of an electronic, microprocessor controlled television tuning system capable of acquiring channels offset from the nominal frequency is disclosed in U.S. Pat. No. 4,280,140 issued to Peter C. Skerlos. In this system, AFC discriminator information is provided to the microprocessor for the generation of digital outputs for controlling stepwise tuning adjustments. This information is generated by a pair of window comparators, edge/direction comparator, and a vertical sync detector. When a channel is selected, the tuning system first synthesizes the correct frequency for that channel. Then the microprocessor first looks at the output of the edge/direction comparator to determine the direction of tuning. Once the proper tuning direction is determined by the edge/direction comparator, the microprocessor then steps the AFC tuning voltage in predetermined increments until the limit of the number of allowed steps for that particular channel is reached. When this limit is reached and no carrier is found, tuning is switched and the voltage stepping procedure is initiated again. For a detected carrier to be valid, the output of the window comparators should be high and the vertical sync detector should produce a train of 60 Hz pulses, indicating the presence of vertical sync information. Although one would expect the Skerlos system to work well, in the highly competitive television market the cost of implementing such a system is objectionable.